The present invention relates to a switching device and a method for assignment of time slots to a multichannel link.
Switching devices in communication systems are used to set up, control and clear down connections for transmission of useful information. In the simplest case, the switching device has a coupling stage which connects a subscriber A to a subscriber B. The connections of the subscribers A and B may in this case be located at one port or different ports.
It is furthermore known that a switching device can implement a plurality of connections, using the so-called time division multiplex mode, via a single connecting line. Such a physical connecting line can transmit a plurality of channels which are formed by time slots in the time division multiplex mode. As a result, there is no hard-wired physical connection in a coupling stage in a switching device which is operated using this principle, time slots being assigned for a connection, instead.
If a connection is considered from a subscriber A to a subscriber B via a switching device, the connection being regarded for simplicity as being unidirectional, then incoming time slots from the subscriber A must be assigned in the switching device to outgoing time slots from the switching device to the subscriber B. The information which is transmitted during the time slots is thus switched. In the case of the time division multiplex mode, the time slots are organized on the basis of frames, that is to say one frame contains K time slots, which can each be assigned to different connections. The same time slot in each case designates an existing connection from frame to frame.
A bit rate, for example 64 kbit per second for B channel in ISDN switching devices is assigned to each of the channels formed by a time slot. If a connection from the subscriber A to the subscriber B now requires a higher bit rate, then a multichannel link is required. This just means that a plurality of time slots must be assigned to the connection within a frame. However, in this case, it is not necessary for these time slots to be transmitted on the same physical connecting line.
In comparison with single-channel links, additional difficulties occur in the case of multichannel links. Normally, a continuous information stream is produced by the subscriber A. The information elements in this information stream are arranged in accordance with a specific sequence. This sequence must also be ensured at the subscriber B, after transmission of the information. The subscriber's communications terminals and the switching device satisfy this requirement. It is desirable that such problems be kept away from the communications terminals and that the handling of multichannel links in accordance with the requirements be provided in the switching devices.